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    <meta charset="UTF-8">
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    <title>Document</title>
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<script>
    //定义变量,变量不给值undefined

    var a;

    //NAN：不是数字
    //a = '12.1';
    //现在需要实现四则运算
    // parseInt ：解析成int类型
    //parseFloat():解析成写小数
    console.log(a + 12);

    console.log("你好")

</script>
</html>